The caliber describes (more or less) the diameter of the bullet. Calibers measured in inches begin with a decimal (e.g. .22, .357, .45, on up through light artillery). Calibers measured in millimeters are, naturally, described as such (9mm, 20mm, etc.) That said, there is a lot of superfluous nomenclature.For example, the .30–06 is the same diameter as the .30–30, but they have different case lengths and powder loads. The .22, the .223, and the 5.56mm bullets are nearly identical (just .003 inches difference between the first and the rest), but have different case lengths and powder loads. The .38 Special is actually .357in. in diameter, but has a different casing/powder setup as the bullet that you actually call a .357, and so on.

@Jamspoon
Indeed, considerably larger, and I think that the bullets are a bit longer and pointier, too.
The .223 and the 5.56 are similar enough that they can both be used in a Ruger Mini-14 (near as I can tell, any gun chambered for 5.56 will fit the .223 without any trouble, but the reverse isn’t true).

The caliber relates to the size and has to do with the velocity with which it kills you. .17 caliber rifle is for shooting varmint but can kill you from far away because it’s a high-velocity round, but it is the smallest rifle bullet. The biggest bullet is .500, a ‘500,’ and the .416 is the biggest round that’s flat shooting. Courtesy of my sometimes better-half Billy Z.

@katwalk65
Almost :D.
The Denel pitch video describes the gun as being useful for disabling pretty much anything that isn’t heavily armored. The rounds come in many flavors, including HE and armor-piercing.

Technically, you could shoot somebody with it. But talk about your overkill…
More practical (and less expensive!) would be to use the included mod kit (a new barrel, bolt, and magazine) to scale the weapon down to 14.5mm or even the 12mm training rounds.

“I like to see girls of that… caliber. Dr. Evil: By “caliber,” of course, I refer to both the size of their gun barrels and the high quality of their characters… Two meanings… caliber… it’s a homonym… Forget it.”

No, two different names for the same round. The early military loads (mid 1960s) used a different type of powder than the commercial Remington .223, but they’ve been ballistically identical for the last 40 years. The early military loads had caused fouling and jamming problems, resulting in the “forward assist” plunger to be added to the M16A1. The ultimate correction was using a powder like the commercial Remington loads that burned cleaner.
There are various commercial loads available in .223 Remington, but the standard 55g FMJ load is identical to the 5.56 NATO.

The force of a fired bullet is determined by how much and what type of propellant it used, the caliber of the bullet, what material the barrel is made of, whether or not it is rifled, if so, in what fashion, the length of the barrel, whether or not there was any fouling inside of the barrel at the time the bullet was fired, the age of the propellent, etc, etc.

There are a ton of variables involved in figuring how powerful a fired shell is.
-Dan

As for figuring out how powerful a particular gun is, you have to also specify the ammo. Muzzle energy depends on the factors that @Dan_DeColumna mentioned, and how much energy it has at range depends on the range, the aerodynamics and mass of the round, and atmospheric variables (temperature, barometric pressure, wind velocity…).
Generally, bigger rounds do better at range partly because they have more powder behind them and partly because they have more mass for the same cross-sectional area thanks to the “Square-cube law” so they decelerate less quickly. That is part of why a Desert Eagle .50 AE can’t compare to a .50 BMG despite them being the same diameter.

Also, barrel length plays a pretty big role, so even if you know the propellant energy of the ammunition used in your old man’s .45, than still wouldn’t help much unless you also took into account the gun itself.

And yes, the 88mm AAC uses a big round, though it’s more of a shotgun shell.

Now for the confusing part!!!

While small-arms use “caliber” to refer to the diameter of the round, artillery uses the term “bore” for that and uses “caliber” to describe the ration of barrel length to bore. For instance, the main guns on the USS Missouri are also “50 caliber” because their 800” (or 66’8”, or 20.3meter) barrels are 50 times their 16” bore.

Good explanation @jerv . Additionally, the bore or “gauge” of a shotgun is based on an old formula of how many lead balls the diameter of the bore equal one pound. So a 12 gauge shotgun bore is equivalant to the size of a one-twelfth pound lead sphere. Also, the British extend this system into artillery; a 12 pounder being a gun bore equal to the diameter of a 12 pound lead sphere.

In the late 19th century, the term “Nitro” was added to some cartridges to distinguish them from black powder loads.

The term “magnum” is also used to distinguish a more powerful version of another cartridge, usually the cartridge case made slightly longer so as not to fit a weapon not designed for it; for example .38 Special/.357 Magnum and ,44 Special/.44 Magnum; the lower powered loads can chamber and fire in the higher rated weapon, but not vice-versa.

It goes almost without saying (again) that using the metric system can confuse people. For instance, the .50 BMG round is also called “12.7×99mm NATO” even though it’s the same thing (and different from the Russian 12.7×108mm round).

Of course, you don’t see that much unless it is a round used by both the US military and foreign gunmakers, but it still throws people for a loop sometimes.